The German company Ortlieb produce a range of waterproof items. These are excellent for use while trekking, motorcycling, bicycling, caving, canoeing, etc. I usually put clothing and sleeping bags in Ortlieb dry bags inside my rucksack. I am guaranteed that stuff will keep dry, and it makes it easier to organize the backpack.

I also have a larger Ortlieb bag which I use when I go on motorcycling trips. Useful stuff, and excellent quality/durability. They produce a range of items.